A Simple Strategy for the Controlled Synthesis of Ultrasmall Hexagonal-Phase NaYF4:Yb,Er Upconversion Nanocrystals

Graphical It′s all under control : Ultrasmall NaYF 4 :Yb,Er upconversion nanocrystals have been prepared by the simultaneous addition of two methanolic solutions of NH 4 F and NaOH into a rare-earth-salt precursor solution. A large number of hexagonal-phase seeds are quickly formed as a result of the low energy barrier, resulting in an ultrasmall particle size. Precise control over the size and crystallographic phase of nanocrystals allows the manipulation of their properties and applications as desired. Hexagonal-phase NaYF 4 (β-NaYF 4 ) upconversion nanocrystals with efficient luminescence are quite desirable for bio-applications, but synthetic control over the nanocrystallite size remains challenging. Herein we verify experimentally that control of the hexagonal-phase nucleation is crucial for controlling the final particle size and the number of hexagonal-phase seeds strongly depends on the actual F(Na):Y ratio. A negligible number of hexagonal-phase seeds were formed at low ratio whereas the high ratio yields a very large number of seeds. We also identify that the actual F(Na):Y ratio is more likely related to the different methods employed for the addition of NH 4 F and NaOH rather than its initial feeding ratio. By taking advantage of this dependence and varying the reaction conditions (temperature and time), β-NaYF 4 :Yb,Er nanocrystals with a size smaller than 6 nm can be synthesized. Theoretical modeling shows a good agreement with the experimental results.